The present invention relates to the use and manufacturing process of telescope mirrors for high bandwidth free space optical data transmission.
The increased need for high bandwidth (high data rate) communication links induced by the recent growth of the internet and other telecommunication means lead to renowned interest in the free space optical data transmission (Whipple, “Free space communications connects”, Photonics at work, October 1999). In free space optical communications the data are transmitted through a communication link between a transmitting station to a receiving station by a laser being preferably having a frequency of about 1550 nm without using a physical medium like an optical fibre or the like. Depending on the weather conditions communication links over a distance of several kilometers with a bandwidth of up to 2.5 Gb/s have been demonstrated (P. F. Szajowski et al, “Key elements of high-speed WDM terrestrial free-space optical communications systems”, SPIE Paper No. 3932-01). Such free space optical telecommunication links are especially useful for connecting facilities having high data transmission needs like banks or universities in metropolitan areas with one another. Another possible application is the live and high bandwidth broadcasting of sports events, where an optical free space communication link can be set up temporarily with low costs.
In order to avoid health risks by the laser radiation the laser power has to be low (a few milliwatts) and the beam diameter must be large, about several 10 centimeters. To establish an optical free space communication link the optical signal therefor has to be coupled out of an optic fibre network and directed with a transmission telescope over the desired distance directly to the receiving telescope where the received beam has to be concentrated and coupled into an optical fibre network. The reliability and the achievable free space distances depend on the efficiency of the transmitter as well as the receiver telescopes. It is known to use high precision glass or zerodur mirrors as reflective optical elements of the transmitter as well as the receiver. These are, however, expensive to manufacture with the required precision.
There is therefore a need for cheap, reliable high precision optical elements for free space optical data transmission.
For space based X-ray telescopes reflection grating assemblies have been developed which contain up to 58 tubular shaped X-ray mirrors for concentrating the X-rays to a CCD camera. The X-ray reflectors have a combined paraboloid/hyperboloid geometry (Walter optics) and are manufactured by a nickel electroforming process using a mandrel for defining the reflector geometry (D. de Chambure, et. al, “XMM's X-Ray telescopes”, esa bulletin 100, December 1999; A. Valenzuela, “Precision optics by large area replication”, Proceedings 34th Liège International Astrophysics Colloquium ‘The next generation space telescope: science drivers and technological challenges’, Liège Belgium, 15-18 Jun. 1998 ESA SP-429, October 1998; D. de Chambure, et. al “Producing the X-ray mirrors for ESA's XMM spacecraft”, esa bulletin 89, February 1997; R. Graue, et. al, “Jet-X mirror assemblies-galvanoplastic technology and high energy performance”, 47th International astronautical congress, Beijing, China, Oct. 7-11, 1996).